Knowledge Bank

Rotationally resolved near-infrared spectra of the chloromethylene (HCCl) radical $A~1A\prime \prime \leftarrow X~1A\prime$ transition were obtained at Doppler-limited resolution using a combination of laser transient absorption and radio frequency modulation (FM) techniques. Merer and $Travis1$ first reported the observation of the HCCl $A~\leftarrow X~$ bands, but were unable to rotationally resolve the $A~(0,1,0)\leftarrow X~(0,0,0)$ band and did not observe any other near-infrared bands due to their extreme weakness. $Kakimotoetal.2$ reported several laser-induced fluorescence (LIF) studies on the HCCl $A~\leftarrow X~$ bands at visible wavelengths, which correspond to the transitions terminating in the levels well above the barrier to linearity in the $A~$ and $X~$ states. In the present study, we have identified the transitions from the $X~(0,0,0)$ level to the $A~(0,0,0),A~(0,1,0)$, and $A~(0,0,1)$ levels. The $A~(0,0,0)\leftarrow X~(0,0,0)3$ and $A~(0,0,1)\leftarrow X~(0,0,0)$ bands had not been previously observed. These observed $A~$ state levels are located below the barrier to linearity; i.e., the geometry in these levels is bent. Analysis has determined the geometry for the $A~(0,0,0)$ level: $\angle HCC1=134.7\circ$ and $R(C-Cl)=1.623\backslash AA$. We have also found that the rotational structure is still perturbed by Renner-Teller effects resulting from interactions between the $X~$ and $A~$ states which become degenerate at the linear configuration in a combination with spin-orbit interactions. A dispersed fluorescence experiment following excitation of a strong $A~\leftarrow X~$ transition in the visible is also in progress. The data from dispersed fluorescence spectra will shed light on the vibrational structure in the $X~$ state and probably will help to determine the singlet-triplet $(X~1A\prime -a~3A\prime \prime )$ separation. The details of our experimental data and their analysis will be presented. Acknowledgment: This work was carried out under Contract No. DE-AC02-76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.